This invention relates to a method of fixing contaminated soils in asphaltic concrete and the concrete manufactured by the method.
The essence of a "concrete" is viscosity or thixotropicity. At the time of application it has to be quasiliquid, like a slurry, or at least a material with a pasty plasticity that can be deformed by mechanical operations into the shape in which it is to be used. It has to be able to adhere to a surface if it is a coating, and it has to set up into a degree of hardness that is suitable for the kind and class of operation.
Unlike those concretes which are used as structural elements, asphaltic concrete is generally applied as a layer on a nearly horizontal surface or as a low structure such as a curb or "speed bump". As a regulated component in the governmental infrastructure, asphaltic concrete is subject to very stringent specifications down to the particle size distribution of the aggregate, but for private and commercial use quite satisfactory performance can be achieved with a considerable variation in formulations. Asphaltic concretes containing additives like rubber can be made for special uses like tennis courts and running tracks.
The use of asphalt as an adhesive has a very long history. Some of the most primitive artifacts, like spears and arrows, used naturally-occurring asphalt and natural fibers to attach the stone point to the wooden shaft. It has also been used as a preservative by means of encapsulation--asphalt is found in the formulary of Egyptian mortuaries of the classic period.
Asphalt is produced as a joint product with gasoline and oil in the refining of crude oils. In principle, crude oil could be reconstituted by mixing asphalt, oil, kerosene, gasoline, and naphtha in the proper proportions. In practice, asphalt and the other petroleum products are miscible, if not quite in any proportion, at least over very wide ranges of proportions. If asphalt and kerosene are mixed, a dirty-brown kerosene, or a gummy asphalt, or an intermediate oily substance can result; but within wide variations the mixture does not divide into phases. A small proportion of petroleum product mixed with asphalt merely produces asphalt of a slightly different specification or characterization.
Turning asphalt into concrete involves producing a material that is plastic when it is applied and hard when it sets up, and there are two conventional ways of doing that. They are, for appropriate reasons, called the "hot mix" and "cold mix" processes.
In the hot mix process, the asphalt is liquified by heating. As in most things, the viscosity of asphalt decreases with increasing temperature and it is a liquid at a temperature sufficiently below its flash point to be safe to use. The melted asphalt is mixed with aggregate and kept hot during the mixing stage It is then transported, still hot, to the workplace, or it is stored in heated silos until it can be transported.
Obviously this hot liquid asphalt cannot be mixed with cold, wet aggregate. If the aggregate is cold enough it will immediately cause the asphalt to cool, producing a solidified mass of asphaltic concrete where a plastic mass is desired. If the aggregate is wet there will be explosions of steam when it mixes with the hot asphalt. Hot mix plants, therefore, incorporate a rotary kiln to dry the aggregate and heat it to a few hundred degrees F. It then mixes with the asphalt without difficulty.
The use of hot mix asphalts with contaminated soil has been described, see Czarnecki, R. C., Making Use of Contaminated Soil, Civil Eng., December 1988, pp. 72-74. When the aggregate is soil contaminated with petroleum products the kiln drying process introduces a number of problems, or at least constraints. The conventional asphalt plant drying kiln is heated with an open flame directed into the rotating barrel from the outlet end. Obviously the first constraint must be that the contamination level is sufficiently low so the open flame does not ignite the petroleum or cause the vapors to explode.
As the aggregate progresses down the rotary kiln it gradually heats up to the 500.degree.-800.degree. F. that it reaches at the outlet of the kiln. This produces a gradual distillation of the contamination, so that the light ends are driven off near the entry to the kiln, where there may be no open flame. They are then typically drawn off by exhaust fans, and pass through the air pollution control system (usually a cloth filter or "baghouse"), and are exhausted into the atmosphere. Since these unburned hydrocarbon fumes are not collected by the baghouse, they can contribute significantly to the air pollution produced by the hot mix plant.
As the contaminated soil proceeds down the kiln, the heavier components are distilled off. If these are not ignited by the open flame heater, they will be drawn off by the exhaust fans and cooled in the plenum of the baghouse. There they can combine with the airborne particulates ("fines") and produce a kind of asphaltic concrete that adheres to the baghouse filters, making them difficult to keep clean. Those heavy components that remain with the aggregate are no problem because they combine with the liquid asphalt in the mixing part of the operation.
In the cold mix process, when the concrete is mixed, the asphalt is in the form of an emulsion in which the particles of asphalt are kept suspended in the liquid and separated from each other and the aggregate by a film of water. Under pressure, the film of water is expelled and the asphalt comes into contact with itself and the aggregate. In the process it cements the aggregate into a hard concrete that is essentially identical to the hot mix bituminous concrete.
Some of these emulsions incorporate oils as well as the usual asphalt, water, and emulsifying agents because the oil serves to soften the asphalt and make for better adhesion to other additives and fillers.
The use of cold mix asphalts with contaminated soils has been described, see Eklund, K., Incorporation of petroleum-contaminated soils into bituminous concrete, petroleum contaminated soils, V-1, Lewis Publishers, Chelsea, Mich. Although attempts have been made to incorporate hydrocarbon contaminated soils into cold mix concretes, the results have not been satisfactory. In order to use asphaltic concrete, either federal or state regulations must be complied with. These regulations pertain both to the physical properties of the material to be used--whether its a base layer, intermediate layer, or top coat and the chemical properties, the components of the asphaltic concrete must not leach. The drawbacks in the use of the contaminated soils are first the soil itself may range from stone, sand, silt to clay and thus have a wide range of sizes, including rocks, etc. Secondly, the soil may contain in varying amounts, gasoline, lubricating oils, and Nos. 2 through 6 fuel oils in any combination. This requires that a soil and chemical analysis be made. If the contaminated soil is to be used, then the other standard components used in the process for making asphaltic concrete must be adjusted each time in order to accommodate each different batch of contaminated soil to produce an asphaltic concrete that meets specification. More importantly, because of the uncertainty of the precise nature of the chemical contaminants, at the minimum a sample batch of bituminous concrete must be made and tested. Even if the sample meets specifications re: leaching, when the process is scaled up for a production run, due to the normal problems encountered in such a scale up, there still is not complete certainty the final product will meet specifications. At least for these reasons, the use of contaminated soils in cold mix asphaltic concrete has not gained commercial acceptance.
We have discovered a process and the product of the process that uses contaminated soils for the manufacture of cold mix asphaltic concrete. Our process "fixes" the hydrocarbons such that they do not leach from the final product. This fixation is consistent regardless of either the hydrocarbon mix or the soil mix. After an initial analysis of the contaminated soil, it is then used directly in production runs.